1. Field of the Invention
The present invention relates to methods and apparatus for transferring frames of data from a first local area network through a bridge to a second local area network; and more particularly to N-way bridges for internetworking among source routing networks, such as token rings.
2. Description of Related Art
The token ring access method specified by the ANSI/IEEE Standard 802.5 has gained widespread application in the industry. One common technique for interconnecting such token ring networks is known as the IBM Source Routing specification (Token Ring Network Architecture Reference, 3rd Ed., IBM Corp., 1989, (SC 30-3374-02), pp. 2-6 to 2-11).
The IBM Source Routing specification has been designed for two-way token ring internetworking bridges. The internetworking industry, however, has grown to require bridges capable of interconnecting more than just two token ring networks. This technique is known as N-way bridging. Because of the original limitation to two-way source routing bridges, vendors have come up with a number of work around schemes to perform N-way bridging. Vendors of which Applicant is aware to date use a scheme requiring a "virtual ring" node within the bridge to work around the limitation to two-way bridging in the Source Routing specification. The use of a virtual ring node has a number of disadvantages that arise out of the limitations of source routing in token ring networks.
The manner in which source routing information is formatted in standard frames leads to at least one limitation. The IBM Source Routing format uses three basic fields within a source routed frame to perform the bridging function. These fields are often referred to as ring in, bridge number, and ring out. The ring in parameter includes the ring number of the input LAN, the bridge number includes an identifier for the bridge, and the ring out parameter includes the ring number of the output LAN. The bridge source routing hardware looks at the ring in, bridge number, and ring out parameters to determine whether the frame matches the input ring, output ring, and bridge number of the local two-way configuration. If the values match, then the frame is copied up to software in the bridge host processor for retransmission. If the fields do not match, then the frame is not copied.
Source routing is based on the development of a routing information field which includes one or more ring in, bridge number, ring out sets which are configured into a plurality of segment number fields in the source routed frame format. In the source routing segment, the segment number fields are 16 bits wide and contain a ring number and a bridge number. These segment number fields can be concatenated up to eight times according to one standard to provide an internetworking route with a maximum of eight hops from one local area network through a bridge to another local area network. There are ways to extend this hop count limit at the expense of additional complexity of source routed frames.
Using the virtual ring technique consumes one of the available hops. This occurs because in order to send a frame through an N-way bridge using the virtual ring technique, the source of the frame must include a segment number for the virtual ring on the bridge. Thus, to route a frame through a bridge, the frame must store the ring in identifier of the input ring, the bridge number of the bridge, the ring out identifier of the virtual ring, the bridge number of the bridge, and the ring out identifier of the actual output LAN. As can be seen, this technique consumes three segment numbers for a single hop. Thus, in an internetworking configuration, which includes a number of virtual ring type N-way bridges, the physical hop count that can be accomplished in a source routed frame is substantially reduced. Also, for very large internetworking configurations in which the maximum number of local area networks is being approached, the virtual ring type N-way bridge is undesirable because it consumes a local area network number for the virtual ring identifier.
It is desirable to provide for N-way bridging in token ring networks in which source routed frames are transmitted without suffering the penalty of the virtual ring technique, while maintaining hardware filtering available for the IBM Source Routing specification.